The present invention relates generally to hydraulic lash adjusters, and more particularly to a hydraulic lash adjuster (HLA) of the type in which there is both a high pressure chamber and a reservoir, or low pressure chamber.
Hydraulic lash adjusters (also sometimes referred to as "lifters") for internal combustion engines have been in use for many years to eliminate clearance, or lash, between engine valve train components under varying operating conditions, in order to maintain efficiency and to reduce noise and wear in the valve train. Hydraulic lash adjusters operate on the principle of transmitting the energy of the valve actuating cam through hydraulic fluid, trapped in a pressure chamber under a plunger. During each operation of the cam, as the length of the valve actuating components varies as a result of temperature changes and wear, small quantities of hydraulic fluid are permitted to enter the pressure chamber, or escape therefrom, thus effecting an adjustment in the position of the plunger, and consequently adjusting the effective total length of the valve train.
The cam operating cycle comprises two distinct events: (1) operation on the base circle and (2) valve actuation. The base circle event is characterized by a constant radius between the cam center of rotation and the cam follower, and during this event, no cam energy is transmitted. The valve actuation event is characterized by a varying radius between the cam center of rotation and the cam follower, which effectively transmits cam energy to open and close an engine valve. During the valve actuation event, a portion of the load resulting from the valve spring, the inertia of valve train components, and cylinder pressure are transmitted through the valve train and through the lash adjuster. The load increases the pressure of the hydraulic fluid within the lash adjuster pressure chamber, in proportion to the plunger area, and in typical hydraulic lash adjusters currently in commercial production, fluid escapes the pressure chamber between the plunger and the wall of the lash adjuster body. Such a device is referred to as a "conventional leakdown" lash adjuster.
As the fluid escapes, the volume of the pressure chamber is decreased and the plunger moves down, shortening the effective length of the lash adjuster. During the base circle event, the lash adjuster plunger spring moves the plunger up within the body such that no clearance or lash exists between valve actuation components. As this occurs, hydraulic fluid is drawn into the pressure chamber through the plunger check valve in response to the increasing volume of the pressure chamber as the plunger moves up. If the effective length of the valve train shortens during the valve actuation cycle, positive lash is created and the lash adjuster extends, moving the plunger to a higher position at the end of the cycle than at the beginning. Conversely, if the effective length of the valve train increases during the valve actuation cycle, negative lash is created and the lash adjuster contracts, moving the plunger to a lower position at the end of the cycle than at the beginning. The latter condition typically occurs when valve train components lengthen in response to increasing temperature.
As noted previously, commercial lash adjusters of the conventional leakdown type have controlled the escape of fluid (or "leakdown") from the high pressure chamber solely by the fit of the plunger within the body, thus necessitating close clearances therebetween, and selective fitting of the plunger to the body. In an effort to overcome the expense of such selective fitting of the plunger within the body, as well as other associated disadvantages of the prior art, the assignee of the present invention has developed a lash adjuster in which the leakdown flow is past the plunger check valve. This improved lash adjuster is illustrated and described in U.S. Pat No. 5,622,147 for a "HYDRAULIC LASH ADJUSTER", assigned to the assignee of the present invention and incorporated herein by reference.
In lash adjusters of the type disclosed in the above-identified patent, fluid which exits the lash adjuster to lubricate the interface of the ball plunger and the rocker arm must be replenished from a source of low pressure fluid, which is typically the engine oil pump. Therefore, in the conventional lash adjuster, the body has a fluid port machined radially through its wall, with an annular collector groove formed about the inside diameter of the body. Also, the upper plunger member has a radial bore machined through its wall, in a location such that the plunger bore communicates with the collector groove. Fluid then flows inward through the body bore and collector groove, then through the plunger bore, into the low pressure chamber.
Although the fluid flow path described above results in a lash adjuster which performs in a satisfactory manner, the need for the collector groove on the inside diameter of the body, or alternatively on the outside diameter of the plunger, and the radial bore in the upper plunger adds substantially to the overall manufacturing cost of the lash adjuster. The presence of a groove on the body inside diameter is undesirable because the ring seal mounted on the lower plunger must traverse the groove during lash adjuster assembly, and in so doing, the seal may be damaged. In addition, the length of the upper plunger must be sufficient to provide adequate contact area above and below the groove to minimize wear, and must be sufficient above the groove to minimize oil leakage through the clearance between the upper plunger and the body. Within a given body bore depth and a given range of plunger axial motion for a specific engine application, the length of the lower plunger is directly influenced by the length of the upper plunger. The lower plunger length may be sufficiently shortened such that, in the case of a conventional leakdown lash adjuster, the leakdown clearance may need to be tightened to compensate, further increasing manufacturing cost.